The invention discloses a preparation method of solid polymeric ferroaluminum sulfate, comprising the following steps: (1) basic ferric sulfate is prepared from ferrous sulfate; (2) water, sulfuric acid and hydrogen peroxide are added into the basic ferric sulfate in sequence, stirring and standing; (3) an aluminized compound is added and further stirred; (4) being self-cured and precipitated under dry condition; and (5) being aged at room temperature and crushed so to obtain a product. The invention uses a blast oxidation process and solves the pollution problem which is generated by the catalytic oxidation process in the traditional iron-based coagulant industry. The polymeric ferroaluminum sulfate is a solid product; compared with the liquid product, the polymeric ferroaluminum sulfate has advantages of storage and transportation, has long service life, and does not need evaporation or decompression evaporation process, and the coagulation active component of the product is retained to the largest extent. Quality indicators such as basicity, active component, and the like are all higher than the quality indicators in the prior art. When in actual implementation and embodiment of water body, the effect of the polymeric ferroaluminum sulfate is better than the effect of the inorganic coagulant such as polyaluminum chloride polymeric ferroaluminum sulfate and the like.